Photoinduced grating in B2 O3 containing glass

ABSTRACT

Boron containing glasses are sensitive to radiation in the band 225-275 nm and therefore, B 2  O 3  glasses are particularly adapted to receive refractive index modulation, e.g., to make reflection gratings. Glasses containing SiO 2  and B 2  O 3  are particularly suitable when the grating is to be localized in the cladding of a fibre. Glasses containing SiO 2 , GeO 2  and B 2  O 3  are suitable when the grating is in the path region of a waveguide, e.g., in the core of a fibre.

This application is a divisional of our parent application Ser. No.08/302,931 filed Sep. 22, 1994 as a National stage application under 35U.S.C. §371 of PCT/GB93/01321 filed Jun. 24, 1993. This application isalso related to our earlier divisional application Ser. No. 09/372,038filed Aug. 11, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical devices which include refractive indexmodulation, e.g. reflection gratings.

2. Related Art

Reflection gratings are often implemented as waveguides which have apath region and/or a confining region with a modulated refractive index.The waveguiding structure is often in the form of a fibre. Themodulation preferably takes the form of alternate regions of higher andlower refractive index. When radiation traverses the modulation, it isselectively reflected. The period of the refractive index modulation isusually equal to the wavelength to be reflected or to a multiple orsub-multiple of said wavelength. Thus periods in the range 250 to 600 nmpreferentially reflect selected wavelengths within the range 800-1650nm.

Reflection gratings have many applications in optical signalling. Forexample, a reflection grating can be associated with a fibre laser inorder to narrow the lasing bandwidth. When the refractive index bandsare not perpendicular to the fibre axis, the grating can be used for theselective removal of unwanted wavelengths. In addition to reflectiongratings, refractive index modulation has other applications, e.g. toachieve phase matching in waveguides, to control spot size and/or shapein waveguides and for storing information.

Refractive index modulation is conveniently produced by an opticalprocess in which a photosensitive glass is exposed to radiation whichcauses an adequate change in its refractive index. The radiation hashigher and lower intensities corresponding to the intended pattern ofmodulation of the refractive index of the glass. In many commonly usedembodiments, the mutual interference of two beams of radiation producesthe variation of intensity appropriate for reflection gratings. In thecase of information storage, the pattern of radiation relates to thedata to be stored.

Silica/germania glasses are widely used in optical telecommunicationsand it has been noticed that these glasses have an optical absorptionband extending approximately over the wavelength range 225-275 nm andexposure to radiation within this band increases the refractive index ofthe silica/germania composition. The peak of the band occurs at awavelength which is close to 240 nm. It has, therefore, been proposed toproduce refractive index modulation, e.g. to make reflection gratings,by exposing silica/germania glass compositions to radiation within thewavelength band 225-275 nm. Radiation close to 240 nm is particularlysuitable. High powers of radiation, e.g. above 1 mW continuous, areneeded to produce adequate changes in the refractive index and writingtimes of a few minutes to a few hours are appropriate.

WO86/01303 describes the writing of phase gratings in optical fibres orwaveguides by the application of intense beams of ultraviolet light. Itis stated that the grating produced in the core of a wave guide and thatthe core is preferably a germanium-doped silica or glass filament.

SUMMARY OF THE INVENTION

The sensitivity of the glass is important, and this invention is basedupon the unexpected discovery that glasses which contain B₂ O₃ areparticularly sensitive to radiation e.g. radiation close to 240 nm, andthat these glasses are well adapted to carry the necessary refractiveindex modulation. Preferably the glass contains at least one of SiO₂ andGeO₂ as well as the B₂ O₃.

Compositions consisting essentially of GeO₂ and B₂ O₃ preferablycontaining at least 2 mole % of each component, are suitable for thinfilm optical devices which are capable of storing data in the form ofrefractive index modulation.

Compositions consisting essentially of SiO₂ and B₂ O₃, preferablycontaining at least 2 mole % of each component, are particularlysuitable for carrying the refractive index modulation wherein saidmodulation constitutes a reflection waveguide located in the confiningregion of an optical waveguide. Glass consisting essentially of SiO₂ andGeO₂ would be particularly suitable for use as the path region of saidwaveguide.

Compositions (herein after called ternary compositions) consistingessentially SiO₂, GeO₂ and B₂ O₃ are particularly suitable for use inoptical devices according the invention. Preferred ternary compositionscontain:

2-40 mole % of B₂ O₃,

2-40 mole % of GeO₂, and

at least 30 Mole % of SiO₂.

It should be noted that B₂ O₃ tends to decrease the refractive index ofa silica glass whereas GeO₂ tends to increase the refractive index of asilica glass.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying FIG. 1 schematically depicts an arrangement for writinga refractive grating into a B-doped fibre according to this invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Since the concentration of B₂ O₃ affects the refractive index as statedabove, the refractive index will display a maxima at minima B₂ O₃concentration and the refractive index will display a minima at maximumB₂ O₃ concentration. It is standard practice in the preparation ofoptical waveguides to vary the concentration of a dopant radiallythrough the core region, e.g., to fabricate a graded index multimodefibre. However, it is less convenient (and even impractical) to producefine detail longitudinal variation, e.g., a reflection grating, byvarying the concentrations of relevant components.

It has been noticed that some glasses are photo-sensitive wherebyexposure to suitable light causes changes in the refractive index andexposure to fine patterns is adapted to produce the desired fine detail.It is doubtful that the optical exposure changes the chemicalcomposition of the glass and it is more appropriate to postulate thatstructural changes, possibly including defect centres, play asubstantial role in the overall effect. Even though the mechanism is notfully understood, the production of refractive index patterns byexposure to radiation has been demonstrated experimentally.

It has already, been stated that glasses which contain B₂ O₃ areparticularly sensitive to radiation and, as indicated above, therefractive index patterns produced in accordance with the invention areindependent of the boron content of the glass. Conveniently the moleratios B:Si and B:Ge are constant in the region where the refractiveindex modulation is applied. In most applications it is appropriate forboth ratios to be constant, e.g., the glass has a uniform composition.(Where one of the elements silicon or germanium is absent it isconvenient to take the relevant ratio as 1:0.

Ternary compositions as defined above have great potential for adjustingthe important properties of the glass as required. The refractive indexis one of the important properties because it is usually of majorimportance to match the refractive indices of the device according tothe invention to the refractive index of adjacent optical components.The device according to the invention is often required to perform awaveguiding function and proper adjustment of the refractive indices ofthe confining region and the path region are necessary to get goodwaveguiding properties. In particular it is important to adjust therefractive index difference between the path region and the confiningregion to a predetermined value. This difference is usually called Δn.

It is possible to adjust the ratio of B₂ O₃ : GeO₂ so that the decreasein refractive index caused by the B₂ O₃ is balanced (approximately orexactly) by the increase caused by the GeO₂. Thus the ternarycompositions with B₂ O₃ in excess of the amount needed to balance theGeO₂ will have refractive indices lower than that of pure silica whereasternary compositions with an excess of GeO₂ will have refractive indicesgreater than that of sure silica. The ternary compositions can be usedin either the confining region, or the math region or in both.

The terms "confining region" and "path region" are used to designate theregions of lower and higher refractive index respectively. It will beappreciated that, especially in the case of single mode waveguides,substantial portions of energy will be transferred in that part of theconfining region which is close to the path region. Thus the energy inthe confining region will interact with a reflection grating located inthe confining region whereby gratings in the confining region can beused either alone or to enhance the effect of gratings in the pathregion.

It will be appreciated that the waveguiding structures mentioned abovemay be either planar waveguiding structures or fibres, especially singlemode fibres. In the case of a fibre the confining region corresponds tothe cladding and the path region corresponds to the core.

In addition to the essential ingredients as specified above the classesused to make optical devices according to the invention may containconventional additives, e.g. melting point depressants to facilitateprocessing during the manufacture of the articles. Melting pointdepressants for silica glasses include phosphorus, usually present as anoxide, and fluorine.

The preparation of optical devices according to the invention usuallyincludes the preparation of the glasses by the oxidation of theappropriate chlorides using O₂ at high temperature as the oxidizingagent. If desired, the glass intended to carry the refracted indexmodulation may be subjected to milk reduction, e.g. by heating in theabsence of oxygen. This is conveniently achieved by heating the glass inthe presence of helium.

The refractive index modulation is applied to the glass which containsB₂ O₃ exposing said glass to the appropriate pattern of radiation whichaccesses the absorption band having a weak close to 240 nm. Radiationhaving wavelengths within the hand 225-275 nm, e.g. a wavelength whichis close to 240 nm, is particularly suitable. Radiation which has doublethese wavelengths is also effective.

Two reflection gratings according to the invention will now be describedby way of example. The gratings are located in the core of a fibre basedon silica glasses and the preparation of the fibre will be describedfirst. The exposure of the fibre to radiation in order to produce therefractive index modulation will also be described with reference toaccompanying drawing.

The fibre was prepared by a modification of the well-known insidedeposition process for making optical fibre. In this process, theappropriate number of layers are deposited on the inner surface of atube which serves as a substrate. Thus the outermost layers aredeposited first and the innermost layers are deposited last. After allthe layers have been deposited, the tube is collapsed into a solid rod,and the solid rod is drawn into fibre.

Individual layers are produced by passing a mixture of oxygen and SiCl₄with reagents such as B₂ Cl₃ and GeCl₄ through the tube and heating asmall section thereof to temperatures in the range 1200° C.-2000° C.Under these conditions the chlorides are converted into thecorresponding oxides which initially deposit on the wall of the tube inthe form of a fine "soot" which is immediately fused to give aconsolidated glass.

As an alternative the deposition is carried out at a temperature suchthat the deposit remains in a porous state and, at a later stage in theprocess, the "soot" is fused at a higher temperature to give theconsolidated glass. This alternative is appropriate when it is desiredto submit the deposit to chemical treatments wherein the porous statefacilitates the desired reaction, e.g. reduction. Melting pointdepressants such as phosphorus and fluorine may he incorporated in themixture to facilitate processing by causing fusing at lowertemperatures.

The heating is carried out by causing a flame to traverse along thelength of the tube. The flame heats a short section of the tube so thata portion, about 20 mm long, is heated to the working temperature. Thistechnique of healing is used for all stages of the process, i.e. for thedeposition, for consolidating porous layers to solid layers and for thecollapse of the tube. Multiple passes are used at all stages of theprocess.

The starting tube was made of pure silica. It had an external diameterof 18 mm and an internal diameter of 15 mm.

Cladding Deposition

The deposited cladding took the form of SiO₂ with phosphorus andfluorine to reduce its melting point. Six layers of cladding weredeposited, and the conditions used for the deposition of each layer wereas follows:

    ______________________________________                                        Oxygen            2       liters/min                                          Helium            1.5     liters/min                                          SiCl.sub.4        0.45    liters/min                                          POCl.sub.3        0.1     liters/min                                          CCl.sub.2 F.sub.2 0.0005  liters/min                                          ______________________________________                                    

In the case of SiCl₂ and POCl₃ the flow rates specify the rate of flowof O₂ through a bubbler thermostated at 24° C. The working temperaturewas approximately 1525° C. It is emphasised, that after each traverse,each cladding layer was in the form of a clear glassy layer before thenext layer was deposited.

The cladding layers could be considered to be part of the substrate tubeupon which the core layers were deposited. The deposition of claddinglayers as described above could be omitted. The main purpose of thecladding layers is to reduce the risk of contamination from the originaltube affecting core layers.

Core Deposition

Core was deposited in two layers and the conditions for the depositionof each of the two layers were as follows:

    ______________________________________                                               Oxygen                                                                              2.0 liters/min                                                          BCl.sub.3                                                                           0.03 liters/min                                                         SiCl.sub.4                                                                          0.1 liters/min                                                          GeCl.sub.4                                                                          0.2 liters/min                                                   ______________________________________                                    

In the case of SiCl₄ and the GeCl₄ the flow rates specify the rare offlow of O₂ through a bubbler thermostated at 24° C. In the case of BCl₃the flow rate is that of the vapour itself at 300° C. and 1 atmosphere.

The working temperature was only 1450° C. but this consolidated the corelayers.

After the preparation described above, the tube was collapsed into asolid rod in the conventional manner using five traverses of the flame.

The solid rod, i.e., the preform for fibre, had a core which containedapproximately 57 mole % SiO₂, 25 mole % B₂ O₃ and 18 mole % GeO₂ givingan RI of 1.462. The cladding, essentially SiO₂, had an RI of 1.458 sothat Δn=0.004. The composition of the glass in the core wassubstantially uniform, i.e., the mole ratio B:Si was 1:2.28 throughoutand the mole B:Ge was 1:0.72 throughout.

The procedure described above, apart from the use of BCl₃ in the core,constitutes an essentially conventional preparation of a fibre preform.

The preform prepared as described above was drawn into fibre of 120 μmdiameter at a temperature of 2,000° C. The fibre was produced at a rateof 18 meters/mi. This fibre is the precursor of reflection gratingsaccording to the invention.

Short lengths of the fibre described above were converted intoreflection waveguides using the technique illustrated in the drawingFIG. 1. In each short length of fibre the core had a uniformcomposition, i.e., as specified for its preform. Before exposure asdescribed below the refractive index of the core was uniform.

A short portion 14 of the fibre 15 was illuminated by a source 10. Thisradiation was, in the first instance, produced by an Ar⁺ laser,frequency doubled to give output at a wavelength of 244 nm. The beamfrom the source 10 was directed onto a splitter 11 so that two beamswere directed onto mirrors 12 and 13. The mirrors 12 and 13 caused thebeams to converge onto the target section, 14. Thus an interferencepattern is produced with alternating regions of higher and lowerintensity. Because the fibre 15 is photosensitive, the region 14(whereon the beams are focused) is affected by the beams and therefractive index increased in the areas of high intensity. Thus areflection grating is produced in the region 14.

It will be appreciated that the spacing of the interference pattern isaffected by the angle at which the two beams intersect one another, andhence the spacing of the grating can be adjusted by adjusting therelative position of the splitter 11 and the mirrors 12 and 13.

Two specimens of this fibre were subjected, to an interference patternto produce reflection gratings A and B. For comparison, a reflectiongrating was prepared from a conventional fibre, i.e. without the boron.This comparative crating is identified as grating X. Importantmeasurements on these gratings and their fibre waveguides are given inthe following table.

    ______________________________________                                        GRATING A        GRATING B  GRATING X                                         ______________________________________                                        Length  2 mm         1 mm       2 mm                                          RI Core 1.462        1.462      1.463                                         .increment.n                                                                          .004         .004       .005                                          Index   1 × 10.sup.-3                                                                        7 × 10.sup.-4                                                                      3.4 × 10.sup.-5                         Modulation                                                                    Grating 99.5%        67%        1.2%                                          Reflectivity                                                                  RIC       25%        18%        0.68%                                         Input Energy                                                                          60J          48J        192J                                          ______________________________________                                    

The "RIC" is the relative index change and it is calculated as [(indexmodulation)/Δn)]×100 (to convert to percentage).

(In optical technology, refractive index matching of components is oftenimportant to avoid unwanted reflections from component interfaces. Thusreflection gratings need to be refractive index-matched to adjacentcomponents and this limits the freedom to adjust the composition tomaximise the photo sensitivity and the grating properties. It is usuallyeasier to obtain index modulation in fibre which has high Δn and the RICtakes this circumstance into account).

The properties of grating X can be compared directly with grating Abecause both gratings are 2 mm long. The most important property of thegrating is reflectivity and in this key parameter gracing A is very muchbetter than grating X (99.5% as against 1.2%). It will be appreciatedthat the length of a grating has a strong effect upon its reflectivityand the longer a crating (other things being equal) the better itsreflectivity. It is, therefore, important that both grating A and X havethe same length.

Grating B has only half the length but its reflectivity is still 67%which is considerably better than grating X even though grating X islonger. The index modulations of gratings A and B are similar (10×10⁻⁴as compared with 7×10⁻⁴). Grating X has a much lower modulation(0.34×10⁻⁴) which is a clear indication that the boron, containing theglasses are more photo sensitive. Grating X has a slightly higher Δn(0.005 against 0.004) so the RIC values emphasise the superiority of thegratings according to the invention.

What is claimed is:
 1. A method of producing an optical devicecomprising:spacially modulating the refractive index of a portion ofsaid optical device formed of a glass which contains B₂ O₃ and at leastone of SiO₂ and GeO₂ so as to produce a pattern of refractive indexvariations, wherein said modulation is carried out by exposing saidportion to a modulated intensity of radiation which accesses theabsorption band having a peak close to 240 nm whereby the intensitypattern of the radiation in said portion is reproduced as a refractiveindex pattern that is independent of the boron content of the glass. 2.A method as in claim 1, wherein the wavelength of said radiation iswithin the band 225-275 nm.
 3. A method as in claim 1 wherein said B₂ O₃is present in an amount of at least 2 mol %.
 4. A method of producing areflection grating in an optical waveguide having a glass path regionand a glass confining region, wherein the reflection grating comprises apattern of alternate higher and lower refractive indices, said patternbeing located in a portion of said path region, said portion beingformed of a glass containing silica doped with B₂ O₃ and GeO₂ whichmethod comprises:exposing said portion to a modulated pattern ofradiation, said modulation comprising higher and lower intensitiescorresponding to the pattern of said reflection grating; said radiationbeing adapted to access the absorption band having a peak close to 240nm and to produce a pattern that is independent of the boron content ofthe glass.
 5. A method of producing a reflection grating in an opticalwaveguide having a glass path region and a glass confining region,wherein the reflection grating comprises a pattern of alternate higherand lower refractive indices, said pattern being located in a portionbeing formed of a glass containing silica doped with B₂ O₃, which methodcomprises:exposing said portion to a modulated pattern of radiation,said modulation comprising higher and lower intensities corresponding tothe pattern of said reflection grating; said radiation being adapted toaccess the absorption band having a peak close to 240 nm and to producea pattern that is independent of the boron content of the glass.
 6. Amethod of producing a reflection grating in an optical fibre waveguidehaving a glass core and a glass cladding, wherein the reflection gratingcomprises a pattern of alternate higher and lower refractive indices,said pattern being located in a portion of said core, said portion beingformed of a glass containing silica doped with B₂ O₃ and GeO₂ whichmethod comprises:exposing said portion to a modulated pattern ofradiation, said modulation comprising higher and lower intensitiescorresponding to the pattern of said reflection grating; said radiationhaving a wavelength within the band 225-275 nm and to produce a patternthat is independent of the boron content of the glass.
 7. A method ofmaking a refractive grating in a glass portion of an optical device,said method comprising:providing said glass portion with a boron-dopedglass content that enhances its photo-sensitivity; forming a refractivegrating in said glass portion by exposing said glass portion tospacially modulated radiation intensity having an input energy levelsubstantially less than that which would have been required to form arefractive grating in the absence of said boron-doped glass.